|
|
||||||||
| Home | About | Products | Data Tools | Order Data | News | Help/Edu | Links | Contact Us |
Raw Data in Stitched Orbital SegmentsCharacteristics of orbital segments Nearly 45,000 individual observations were acquired during the first 30 months of this project. While the primary objective of the project is to produce and distribute the 10-day NDVI composites, there are requirements to provide raw AVHRR data. The raw data have been used to support fire detection, cloud screening, and atmospheric correction algorithm development. Most of the algorithm development has been in support of MODIS and MISR instruments on board the future EOS platforms. As part of the routine processing, the individual observations from ground receiving stations are "stitched" together to form continuous orbital segments. The orbital stitching process is described in the general processing documentation. Figure 1 (247,035 bytes) illustrates the typical daily coverage of land from the ground reception network. For some orbits there are scenes that provide pole to pole coverage, such as over Australia to northern Russia. In other cases the orbit paths over land are short, such as over Alaska. When there is an intervening stretch of ocean between land observations, the correct geographic perspective is achieved by filling the gap with zero-valued data. Figure 2 (252,349 bytes) shows two side by side orbit stitches over Africa and Europe and Figure 3 (71,667 bytes) shows a complete day of orbital segment coverage. A raw data orbit is composed of 5-channel, 10-bit, AVHRR data, at 1.1-km resolution (at nadir). The data are in standard NOAA Level-1b format as described in the NOAA Polar Orbiter Users Guide (Kidwell, 1991). In Level-1b format the data are packed as three pixels (3 times 10-bits) per 32-bit word. As a result there is no data storage efficiency gained by additional data compression. The average size of an orbit is approximately 170 megabytes. Please keep this in mind during transmission. Data Availability The production of stitched orbits coincides with the production of the 10-day composites. The 10-day composites are being produced in sequence starting with April, 1992. The initial plan is to maintain approximately 3 months of daily orbits online in a first in first out rotation. Additional requests for orbits, either already processed and in the archive or orbits not scheduled to be processed in a manner timely to your needs, can be requested from LP DAAC User Services Email: custserv@usgs.gov Tel: 605-594-6116 Fax: 605-594-6963 Selecting orbits Currently, the method for determining the geographic coverage of an orbit is not very elaborate. That development is underway as part of the EOSDIS IMS activity. In the meantime the data organization is very simple. Figure 4 illustrates a typical pattern of the fourteen daily ascending orbits of the afternoon NOAA satellites. The nadir track of the AVHRR has a nine-day repeat cycle and each orbit is approximately 20 degrees of longitude wide at the equator. There is very little overlap between orbits at the equator and significant overlap between orbits at the poles. Each orbit is given a unique scene id. An example is AO11071292204218. The scene id is a combination of important information. The "AO" indicates that the scene is an orbit as opposed to "AL" a which is a single LAC scene that had been recorded by NOAA and transmitted to EDC. "AH" would indicate a single HRPT scene received directly by one of the ground stations. The "11" indicates it is NOAA-11 data. The "071292" is a date field that indicates it was acquired on July 12, 1992. The "204210" indicates the start time of the acquisition as 20 hours, 42 minutes, 10 seconds, mean Greenwich (Zulu) time. The orbits are in a directory structure organized as /satellite/year/month/day. The individual orbits that are available are shown in the day subdirectory. In the day subdirectory there will be up to 14 orbits listed. It is more likely that 13 orbits are listed because at least one orbit per day is generally all ocean data. The orbits will be sorted by time of acquisition, in ascending order. In order to determine the specific geographic coverage of an orbit you will need to understand the relationship of time of acquisition and geographic location of the satellite. The start time is part of the orbit id number. Figure 4 is annotated with the approximate Greenwich (Zulu) time at which the orbit begins. If data of North America are required, all orbits with start time of 17 hours through 00 hours should be selected. If data of Africa are required, all orbits with start time of 10 hours through 16 hours should be selected. These start times are approximations. Orbits must be selected one at a time for transmission. Be sure to have at least 250 megabytes available in your /TEMP directory or the transfer will likely fail.
|
